1. Field of the Invention
The present invention relates to an improvement in an apparatus for sampling clock reproducing usable in a television receiver which is provided with an additional receiver for receiving digital signal superimposed on the television signal during the vertical blanking interval.
2. Description of the Prior Art
Broadcasting of an additional signal of character or pattern by superimposing it on the television signal during the vertical blanking interval is known generally as Teletext system, in France as Antiope system or in Japan as NHK (Japan Broadcasting Corporation)'s system of superimposed broadcasting character. The abovementioned system of NHK's superimposed television broadcasting is the system wherein during each vertical blanking interval, information of character or pattern is broadcast in arbitrary one or two horizontal scanning periods (hereinafter referred to as H) between 16th H and 21st H inclusive and also in arbitrary one or two H between 279th H and 284th H inclusive, wherein n-th H indicates n-th horizontal scanning period.
FIG. 1(a) shows one example of an information signal and a header signal preceding thereto, both being superimposed on a television signal during a vertical blanking interval. As shown in FIG. 1(a), between a pair of horizontal synchronization pulse hs and hs, a header part 1 and a subsequent information signal part 2 are disposed following a color burst cb. Detailed constitution of the header part 1 is shown in FIG. 1(b), wherein the header part 1 comprises:
a clock-run-in signal 3, which is a 16-bit pilot signal, PA1 a framing code signal 4 which is an 8-bit signal, PA1 a packet identity code signal 5 which is a 8-bit signal, and channel code signals 6 and 7.
By such system, the information of character or pattern is sent by a television signal as shown in FIG. 1(a) and FIG. 1(b). In order to receive such a signal, the receiver has a measure of reproducing a sampling clock signal with high accuracy. The receiving system disclosed in the same applicant's preceding application (U.S. Pat. Application Ser. No. 882,087, now Pat. No. 4,202,012, UK Patent Application No. 8237/78, West German Patent Application No. P2811260.9 (disclosed in Offenlegungsschrift) and French Patent Application No. 7807429) has the construction that a doubled frequency is made by slicing the clock-run-in signal, treating the sliced wave i.e., for example, differentiating the sliced wave and processing the differentiated pulse signals to obtain a signal with a frequency twice that of the clock-run-in signal, and applying the doubled frequency signal to a resonance circuit of a high Q value and then utilizing the output of the resonance circuit in reproducing the sampling clock signal. However, such a conventional method takes the way of doubling the frequency after producing digital signals (or pulse signal). Therefore, when incoming signal level is weak or its S/N ratio is poor, there is a high possibility of mal-performance caused by noises in the step of processing the pulse signals.